Disappearing voltage

[iwire4]

Can anyone help?
A customer has a light fitting in a bedroom (5 bulbs) which is switched two ways with a dimmer switch by the door and a switch by the bed.
He states that at some point he switched it on and nothing happened. He assures me they were working at some point before!!!!! He replaced all the bulbs to no avail, hence calling me in. It did seem strange that all the bulbs went in one go.
i opened up the ceiling rose and all wiring seemed sound. I did a voltage test to try and determine when the dimmer switch was closed. I couldn't get any readings at the switching wire at all. There was 240volts at the live loop, which was reassuring.
I disconnected the live to the light and did the test again. This time I got 240volts when the switch was closed and nothing when open ( I assume from that the switching is working properly). It must be the light fitting then. I replaced it with a pendant. Job done I thought. Switched it on and again nothing. On testing again I had the same result as before. The switching wire was dead whether the switch was on or off.
I then removed the bulb. Normal readings resumed - 240volts at switching wire when closed and nothing when open.
i asked the customer if anyone had interfered with the switch wiring or anything else before the light failed. He assures me they were working and then stopped working. It was a while ago though.
Has anyone come across this before or explain what is happening. Is is a neutral issue?
Any help would be very much appreciated.
Ian

 

[Hellmooth]

Dodgy dimmer switch?

 

[iwire4]

Thanks for that. Trial and error. I can try replacing the dimmer switch. Would that give those types of results?

 

[Leesparkykent]

Is the fitting at the end of line or are other lights not working?

 

[Hellmooth]

Could always try a normal switch and see if it works ok, if it does then it would point towards a dodgy dimmer switch, if not then I'd be testing further!

 

[Spoon]

Try replacing the dimmer switch with a normal switch. If the lights work then you know the dimmer is faulty.

 

[iwire4]

It's not the end of the line. Other lights are working.

 

[iwire4]

Ok, looks like the way to go. Cheers..

 

[davesparks]

240V which is there with no load, but disappears when load is connected is better known as a ghost voltage.
Why on earth did you think that replacing the light fitting would fix a problem with the switched live not becoming live?
For a ghost voltage to appear on the switched live then it is most likely that this is no longer connected to where it should be. I'd go with a wire broken off at one of the switches or a faulty dimmer.

 

[Taylortwocities]

What are you testing the voltages with? please say it is a two probe voltage tester or a multimeter.

 

[iwire4]

You live and learn. The 240volts was at the switching wire and done with a proper two prong voltage tester Not an electrical screwdriver which has duped me in the past.
Thanks for the replies. I will go to the switch and go from there. Ghost voltage. One for the memory bank.
Cheers.

 

[Leesparkykent]

Did you test for voltage with line and neutral and line and earth?

 

[iwire4]

I only tested between line and earth

 

[Leesparkykent]

I only tested between line and earth

Well go back and test between line and neutral :6: Lol

 

[iwire4]

Doh!!!!!!!!

 

[davesparks]

You live and learn. The 240volts was at the switching wire and done with a proper two prong voltage tester Not an electrical screwdriver which has duped me in the past.
Thanks for the replies. I will go to the switch and go from there. Ghost voltage. One for the memory bank.
Cheers.

You should be able to discern a ghost voltage from a real voltage with an AVI, assuming you fully understand how it indicates these things.

 

[Guest77]

Can anyone help?
A customer has a light fitting in a bedroom (5 bulbs) which is switched two ways with a dimmer switch by the door and a switch by the bed.
He states that at some point he switched it on and nothing happened. He assures me they were working at some point before!!!!! He replaced all the bulbs to no avail, hence calling me in. It did seem strange that all the bulbs went in one go.
i opened up the ceiling rose and all wiring seemed sound. I did a voltage test to try and determine when the dimmer switch was closed. I couldn't get any readings at the switching wire at all. There was 240volts at the live loop, which was reassuring.
I disconnected the live to the light and did the test again. This time I got 240volts when the switch was closed and nothing when open ( I assume from that the switching is working properly). It must be the light fitting then. I replaced it with a pendant. Job done I thought. Switched it on and again nothing. On testing again I had the same result as before. The switching wire was dead whether the switch was on or off.
I then removed the bulb. Normal readings resumed - 240volts at switching wire when closed and nothing when open.
i asked the customer if anyone had interfered with the switch wiring or anything else before the light failed. He assures me they were working and then stopped working. It was a while ago though.
Has anyone come across this before or explain what is happening. Is is a neutral issue?
Any help would be very much appreciated.
Ian

Gasp .

 

[SJD]

Always suspect a dimmer which in these circumstances - 230V with no load, but no voltage as soon as any appreciable load is connected. It is (usually) a trivial & quick job to remove the dimmer switch & link it out (e.g. stuff the cables into a Wago) and see what happens. Obviously isolating the circuit first. Otherwise it is possible to waste a lot of time imagining perfectly good light fittings are at fault.

 

[Darkwood]

Hi iwire4 ...

Can you please either fill your profile info in or provide us with your experience level and competence.

The reason I ask is your posting in the main forum which with the way your question is asked it is to be assumed you are a qualified practising Electrician.

Can you confirm this as reading through your various replies, you have implied you have not carrying out the correct test procedures, this kind of fault finding should be a simple task if correct procedures are followed.

If you can oblige and express your level and experience we can see if your thread need moving to the appropriate area Thankyou.

 

[telectrix]

Gasp .

THOUGHT YOU'D USED YOUR LAST GASP....... A BIT LIKE THE LABOUR PARTY. :devil2:

 

[richy3333]

THOUGHT YOU'D USED YOUR LAST GASP....... A BIT LIKE THE LABOUR PARTY. :devil2:

Sorry who? Didn't they turn the lights out as they left the room

 

[Marvo]

The waveform that a dimmer supplies can give false readings with voltage and current testing because it's a chopped waveform sometimes with higher frequency than normal electrical supply. As mentioned remove the dimmer and test the circuit with a normal switch. Once you're sure it's fault-free then reconnect the dimmer.

 

[amckay]

Has he been playing diy with any of the other fittings on that circuit and there's a loose N somewhere?

 

[shanky887614]

I only tested between line and earth

should have checked between L-N you could have a broken N at the light

What are you testing the voltages with? please say it is a two probe voltage tester or a multimeter.

a lot of multimeters are affected by ghost voltages, analogue multimeters are not usually affected as much as they put a lot more load on it when they test V.

i would have thought a drummold test lamp would be the easiest to see if it is ghost voltage (no light then its a ghost voltage)

you can get an adapter to put in you mm which will put a load across your test leads

en-us.fluke.com/products/all-accessories/fluke-sv225.html#features

basically the above is useful if you dont have an analogue multimeter

 

[HappyHippyDad]

240V which is there with no load, but disappears when load is connected is better known as a ghost voltage.
Why on earth did you think that replacing the light fitting would fix a problem with the switched live not becoming live?
For a ghost voltage to appear on the switched live then it is most likely that this is no longer connected to where it should be. I'd go with a wire broken off at one of the switches or a faulty dimmer.

Do you really think this would be ghost voltage in this instance? I dont understand how the 'amount' of ghost voltage is determined, but is 240V rather a large figure?
When I did my little, very unscientific experiment with 30m's of 1mm 3 core I got a maximum of 91V, I doubt the OP has 30m's going between the switch and the light. http://www.electriciansforums.co.uk...-odd-request-regarding-phantom-voltage-2.html

 

[davesparks]

I've only ever seen or been told of it mimicking the actual mains voltage.
I've only seen a measured value myself a few times, the above symptoms are typical for what I've been told in the past when I've arrived to find such a fault.
Normally I use my AVI and only get one little red LED when there's a ghost voltage present so rarely put a value to it